Engine throttle control apparatus that control the opening of a throttle valve by driving a throttle valve motor in accordance with the amount by which an accelerator pedal is depressed are known. In a conventional throttle control apparatus, a throttle valve motor is supplied with current in accordance with a signal from an accelerator sensor which detects the accelerator pedal stroke corresponding to the amount of depression of the accelerator pedal. The motor is correspondingly driven to operate the throttle valve to thereby control the intake air flow.
To eliminate deviation between the signal from a throttle sensor which detects the throttle opening of the throttle valve and the signal from the accelerator sensor, feedback control of the motor is performed based on proportional-integral-derivative control (PID control),
Japanese Utility Model Laid-Open No. Sho 60-81241 discloses a throttle valve opening maintaining mechanism for maintaining a throttle valve opening that enables a preset low running speed of a vehicle when the accelerator pedal is in a released state, and a control circuit for controlling the throttle valve opening maintaining mechanism. When a system abnormality is determined, the output of current to the throttle valve motor is discontinued and the opening maintaining mechanism is operated.
The system disclosed in Japanese Utility Model Laid-Open No. Sho 60-81241 enables escape driving at the time of a throttle valve failure by setting a predetermined throttle valve opening. However, because this conventional control method fixes the throttle valve to the predetermined opening at the time of a system failure, problems associated with an inappropriate throttle opening arise. For example, if the predetermined opening is the throttle valve opening needed to maintain a vehicle speed on highways or the like, the predetermined throttle valve opening is excessively great on ordinary (i.e., non-highway) roads. Conversely, if the predetermined throttle valve opening is reduced, it is likely that continued vehicle operation and movement will be quite difficult if not impossible on an uphill road or in other road conditions.